When mixing Pacemaker versions, there are two factors that need to be considered. The first is obviously the package version - if that is the same, then there is no problem.

If not, then the Pacemaker feature set needs to be checked. This feature set increases far less regularly than the normal package version. Newer versions of Pacemaker expose this value in the output of pacemakerd --features:

In this case, the feature set is 3.0.7 (major 3, minor 0, revision 7).

For older versions, you should refer to the definition of CRM_FEATURE_SET in crm.h, usually this will be located at /usr/include/pacemaker/crm/crm.h.

If two packages or versions share the same feature set, then the expectation is that they are fully compatible. Any other behavior is a bug which needs to be reported.

If the feature sets between two versions differ but have the same major value (ie. the 3 in 3.0.7 and 3.1.5), then they are said to be upgrade compatible.

What does upgrade compatible mean?

When two versions are upgrade compatible, it means that they will co-exist during a rolling upgrade but not on an extended or permanent basis as the newer version requires all its peers to support API feature(s) that the old one does not have.

The following two rules apply when mixing installations with different feature sets:

When electing a node to run the cluster (the Designated Co-ordinator or “DC”), the node with the lowest feature set always wins.

The DC records its feature set in the CIB

Nodes may not join the cluster if their feature set is less than the one recorded in the CIB

Example

Consider node1 with a feature set of 3.0.7 and node2 with feature set 3.0.8… when node2 first joins the cluster, node1 will naturally remain the DC.

However if node1 leaves the cluster, either by being shut down or due to a failure, node2 will become the DC (as it is by itself and by definition has the lowest feature set of any active node).

At this point, node1 will be rejected if it attempts to rejoin the cluster and will shut down, as its feature set is lower than that of the DC (node2).

Is this happening to me?

If you are affected by this, you will see an error in the logs along the lines of:

error: We can only support up to CRM feature set 3.0.7 (current=3.0.8)

In this case, the DC (node2) has feature set 3.0.8 but we (node1) only have 3.0.7.